The researchers define a vaginal orgasm as an "orgasm produced simply from movements of the penis in [the] vagina without any additional stimulation."

It found that regularity of vaginal orgasms depends on a number of factors including the length of the penis and the duration of intercourse. But, it concluded that the duration of foreplay was not a factor.

Does size matter?

One-third of the women (34%) said that they were more likely to have an orgasm with a longer than average penis [14.5 centimetres], but nearly two-thirds said that they had no preference.

"Given that the vagina [has a high nerve density] throughout… more thorough stimulation of the full length of the vagina… might result in a more fulfilling experience," the paper's authors write.

But Dr Gemma O'Brien, a reproductive physiologist from the University of New England in Armidale, says the methodology of the findings regarding penis length needs careful consideration.

"Self reporting needs to be done very carefully," says O'Brien. "These things come down to perceptions and that introduces a weakness in the study."

Sex education

Brody and colleagues also found that women who were "taught that the vagina (or the vagina as well as the clitoris) was important for eliciting female orgasm" had significantly greater success in regularly achieving vaginal orgasms.

Even more important, was the ability to mentally focus on vaginal sensations.

The researchers suggest that this ability may itself be "due in part to having been guided to do so by sexual education."

As a result, they argue that "it is incumbent upon sex education, sexual medicine, and wider social policy to be supportive of women's sexual health, which includes being supportive of vaginal orgasm."

O'Brien believes this aspect of the research has the potential to help a lot of women.

"It can be applied to help women who have orgasmic difficulties," she says. "The importance of being able to focus, really shows that the brain is the most important sex organ. If women can be trained to focus on their vaginal sensations, they may improve their orgasmic consistency."

Hidden agendas?

Dr Vivienne Cass, an adjunct professor at Curtin University of Technology in Perth and author of The Elusive Orgasm, worries about the motivations of research that emphasises vaginal over clitoral orgasms.

She sees this emphasis as part of the increasing "medicalisation of sexuality".

"There has been concern among my colleagues who, like me, think that there is enormous pressure to fit in with drug companies to help women get vaginal orgasms," says Cass. "Women who can't achieve vaginal orgasms are treated as sick."

She says if they are treated as sick then drug companies might be able to sell them a pill.

"We have to understand that we are not studying orgasms in a nice, value-free, scientific context."

The study also claims that women who have experienced vaginal orgasms "have a greater satisfaction with their sex life, mental health, relationships with both partners and friends, and life in general."

But Associate Professor Rosemary Coates, also of Curtin University of Technology and president of the World Association for Sexual Health, believes that such assumptions are "reverting back to Freudian assumptions about female sexual responses," saying that "some form of clitoral stimulation is almost always required to trigger orgasm."

The study authors dismiss this view as "clitorocentric" and blame it for the "destruction of human pleasure."

Coates says that such a claim is "nonsense".

"The main pleasurable nerve endings are located in the clitoris and to achieve the full pleasure of orgasm, these nerve endings must be stimulated," she says. "I think these authors have clitoral envy."